1. Introduction
There has been a growing interest in enhancing surface characteristics of the material such
as grain refinement, microhardness, wear and corrosion resistance in recent times. For
engineering components that rely on surface interactions, surface properties play a critical
role. Therefore, it is essential for these surfaces to exhibit high strength and hardness while
maintaining a sound internal structure that provides sufficient ductility and toughness. These
materials are classified as surface composites, which belong to the metal matrix composites
(MMC) family. In surface composites (SC), particle reinforcement is limited to the
uppermost layer of the material, with a thickness of up to 3-5 mm [1].
In recent years friction stir processing (FSP) has emerged as one of the most important
processes for producing surface composites. It is a derivative of the friction stir welding
(FSW) process, involving the use of a rotating tool to plunge and stir the material's surface
as shown in figure 1. Along with stirring action the translational movement of the tool at the
material surface generate enough heat which results in dynamic recrystallization in the FSP
region which further changes the mechanical and microstructural characteristics of material.
[2][3]. This leads to the formation of different zones like stir zone (SZ) at central portion
surrounded by a thermomechanical affected zone (TMAZ) which is formed by excessive
plastic deformation and a heat affected zone (HAZ), it is bought about by the interaction of
material flow and high temperature as shown in figure 2 [4]. The quantity of heat produced
is a function of the machining parameters utilisation, particularly the rotational speed and
travel speed of the tool, as well as the tool's and the modified sample's dimensions.
Therefore, all material changes in FSP technology occur only in the solid phase, and the only
source of heat is friction [5].
The work on composite production by utilising FSP was first published by Mishra et. al.[6].
In their experiment, SiC powder reinforcement was applied directly as a slurry to the surface
of base material (BM). Their findings revealed a composite layer with a thickness of 50-200
mm formed with a uniform distribution through the material. The highest microhardness of
173 (HV) was achieved with a volume fraction of 27% SiC particles. Likewise other
researchers start using on different materials and reinforcements for making surface
composite via FSP like, M. Barmouz et al. [7] uses copper (Cu) with nano SiC particles and
conclude that higher volume fraction of the powder increases tensile strength and decreases
its percentage elongation. Since the first study by Mishra et al., a large number of research
studies on fabrication of surface composite has been recorded, and a large number of
research projects are currently in the process of being carried out by using different materials
such as aluminium (Al) [8], titanium (Ti) [9], magnesium (Mg) [10], copper (Cu), graphene
[11] and different alloys of steels [12].
Fig. 1 - Schematic representation of friction stir processing (FSP) [5]
Generally, SCs are typically made by fusing a ductile metallic matrix with hard ceramic
reinforcement to the required thickness. The successful fabrication of metal matrix
composites reinforced with particulates such as SiC , Al2O3 , B4C , TiO2 , ZrO2. Apart from
the different types of reinforcement used, the safety of SCs is largely dependent on the
correct setting of process parameters of FSP. Narayana et. al. [13] evaluated that by
increasing the number of passes of AA5083 with nano B4C, increases hardness by 51% and
wear properties are also improved. They also reported that the tool rotational speed and FSP
pass counts have a significant impact on the homogenous dispersion of reinforcement
particles. Similarly, Sandeep et. al. [14] vary groove width of AA6061 reinforced with SiC
particles, it found out that by increasing the groove width size the area od SC decreases
simultaneously. Composites reinforced with aluminium matrix are alternately the best
material choice due to their strength, ductility and tough- ness as well as their ability to be
processed by conventional methods. Further, aluminium alloys are most popular due to their
cost effectiveness, abundant availability, extensive variety, good formability, and excellent
recyclability. However, tribological and high-temperature capabilities of these materials are
substandard [15].
SZ
HAZ
BM
TMAZ
Fig. 2 - Schematic diagram of different regions of surface composite
Aluminium matrix composites (AMCs) have become more attractive choice for creating
high-strength components that are lightweight. As a low-weight, high-performance material
has grown more appealing, demands for superior material properties have increased in recent
years for high-tech structural and functional applications in the aerospace, automotive,
electronic packaging, and thermal management industries. The production of AMCs with
certain features is mostly influenced by the evenly distribution of the reinforcement in the
Aluminium matrix [16]. Unreinforced aluminium alloys have weak mechanical and
tribological properties, which can be improved by adding additional ceramic particles such
as various oxides, borides, nitrides and carbides of metals or transition metals. However,
some inorganic substance like fly ash, bagasse ash and bamboo leaves etc are also used as a
means of improving the properties. Their morphologies and tribological characteristics were
significantly enhanced by these reinforcements [17].
Although the AA6XXX series wrought alloys offer a great strength-to-weight ratio with
enhanced mechanical properties such as machinability, weldability, formability, and good
corrosion resistance. Due to the high structural capabilities, it possesses, AA6061 aluminium
alloy is a kind of aluminium alloy that is frequently employed in a variety of structural
applications. The majority of the alloy is made up of aluminium, magnesium, and silicon,
with just trace quantities of copper, chromium, and zinc contributing to its makeup. The
magnesium and silicon provide significant strengthening and hardening effects, while also
contributing to the alloy's excellent corrosion resistance. The copper content in the alloy also
enhances its strength and improves its resistance to fatigue and wear. Additionally, the
chromium content in the alloy improves its resistance to corrosion and oxidation at high
temperatures. They have become increasingly popular for versatile applications such as the
design of armour structures, missile casings, automotive chassis, bridge structures, wings
and fuselages in aircraft, and marine structures [18][19].
This article presents a critical review on the recent developments and trends of FSP in
composite fabrication specifically of aluminium alloys. The factors involved in composite
fabrication via FSP, followed by a description of their separate influence on the properties
of manufactured SCs are discussed. Then followed by a detailed discussion on material
characterization, microhardness and wear properties. Following that, the process's
shortcomings are briefly discussed. Finally, the paper is summarised with some future trends
and a conclusion.
2. Literature Review
2.1 Aluminium as matrix material
Aluminium and its alloy have high strength to weight ratio due to which it is widely used in
automotive and aerospace application for various applications because of its unique
agglomeration properties make it an excellent choice for manufacturing composites [16]. In
order to create metal matrix composites (MMC), a variety of pure metals and alloys,
particularly non-ferrous metals, have been employed as the matrix material. Aluminium and
its alloys are the first class of materials that are frequently used as matrix materials. Generally
aluminium alloys have 4-digit designations as shown in table. The primary alloying element
is indicated by the first digit. The change in the alloy's condition from its original state was
represented by the second digit. In order to distinguish one alloy from another in the same
series, the third and fourth numbers are provided[20]. The list of wrought aluminium alloy
is shown in table below. Similar to wrought aluminium alloy, there are cast aluminium alloys
(designated as A365.0) which are also used for fabrication of composite.
TABLE 1 List of different commercial aluminium alloys used in the manufacturing
industry
S.
Aluminium alloy
Major alloying
Applications
Reference
No.
series
elements
1.
1xxx
Pure Aluminium
Generally used for
[19]
(99%)
nameplates, fan
blades etc.
*(Not much used
nowadays)
2.
2xxx
Copper
Automotive
[21]
wheels, aircraft
parts, and some bio
implants etc.
3.
3xxx
Manganese
Pressure vessels,
[22]
heat exchangers
and storage tanks
etc.
4.
4xxx
Silicon
Filler material
[22]
5.
5xxx
Magnesium
Suitable for marine
[13]
application, aircraft
hydraulic tubes and
various armour
plate etc.
6.
6xxx
Magnesium and
Various structure
Silicon
application,
[23][14]
automotive chassis
and aircraft
fuselage etc.
7.
7xxx
Zinc
Aircraft wings &
[22]
fuselages,
automotive gears
etc.
8.
8xxx
Other elements
Power
[24]
transmission,
automobile radiator
and residential
utensils etc.
Besides this, the 1xxx series which is pure aluminium are previously used for a food
packaging trays but now it is not used in industrial application anywhere. The most used
alloy of aluminium is 2xxx series, in which AA2024 is mostly used. Here copper is the major
alloying element which gives high fatigue strength, surface finish and machinability to the
alloy. Likewise, AA3003 is frequently used in cooking utensils and chemical equipment.
Because of its superiority in handling numerous foods and chemicals, as well as its excellent
corrosion resistance. The 4xxx series of wrought aluminium alloy is not really used, it is
rather used in its cast alloy from. But apart from that it is popularly used for welding wire
and brazing wire applications. Aluminium in the 5xxx series offers exceptional resistance to
corrosion and is thus suited for use in maritime applications. In addition to having strong
welding qualities, it also has a wear resistance that ranges from medium to high. The 6xxx
series is one of the most popular alloy aluminium, it has higher strength, good corrosion
resistance upon heat treatment, good machinability and weldability as well. AA6061 is the
most widely used alloy for structural application. The most widely used alloy for aerospace
application is comes under the 7xxx series. AA7075 is put in use when extreme durability is
required. It is commonly used in aerospace structural components because to its high
strength and low ductility after being alloyed with zinc and a tiny quantity of magnesium.
The 8xxx series is used for those alloys with lesser-used alloying elements such as iron,
nickel, and lithium. Alloy 8090, which contains lithium, has remarkable strength and
modulus, making it ideal for aerospace applications where increased stiffness and high
strength are desired while keeping component weight to a minimum. When fabricating the
composites, it is important to take into account the characteristics of the base alloy. It is
important to have a deeper understanding of various series of aluminium alloy while
fabricating composite. This will help acquire additional ideal results.
2.2 Different reinforcement used for fabrication of MMCs
Although all the matrix material like Al-based, Mg-based, Cu-based and Fe-based etc carries
maximum percentage volume in MMCs. Meanwhile the main role for enhancing the
properties of MMCs are reinforcing materials. Due to the fact that the reinforcing materials
themselves determine how well composites work as a whole, a lot of research effort was put
into finding the most appropriate and effective reinforcement materials. In comparison to the
other kinds, the particulate reinforcements saw the greatest amount of use. This is due to the
fact that they are easily accessible and possess superior dispersive qualities [25][22].
Table 2 Commonly used reinforcements in Al MMCs
Reinforcement
Related properties
Application
SiC
Good thermal properties,
Connecting rod, piston, shaft,
refractoriness, good hardness and
rotors, brake disc, and rubber
stiffness
tyres, etc.
High hardness and specific
Refractory material, abrasives,
strength
and coatings
High strength to weight ratio,
Abrasives, nozzle and ballistic
hardness, chemical resistance, and
armor
Al2O3
B4C
References
[26]
[25]
[27]
good nuclear properties
TiC
High wear resistance and good
Brake disc, rotor, and cutting
strength
tools
[28]
ZrB2
Thermal stability and wear
Structural parts mostly
[29]
resistance
3. Fabrication of surface composite via friction stir processing (FSP)
Various conventional methods exist for producing surface composites, including laser melt
treatment, plasma spraying, liquid phase processing at high temperatures, and mixing metals
in powder form with inert oxides for reinforcement. However, these methods can result in
interfacial reactions between the reinforcement and metal matrix, leading to a deterioration
of the composite properties and difficulties in achieving superfine grain sizes [30]. A
promising alternative is Friction Stir Processing (FSP), which allows the creation of surface
composites with minimal or no reaction between the support and the surface. By considering
the material movement during FSP, we can better understand the composite production
process. In terms of plastic deformation, FSP exhibits similarities to conventional methods,
but with the addition of a particle-based third phase. It is evident that the secondary phase
particles introduced into the matrix during FSP influence the material flow rate, although the
underlying mechanisms often align with those observed during FSP itself [31]. The typical
schematic diagram of surface composite formed via FSP is shown in figure.
Although the exact mechanisms behind the material flow rate modifications caused by the
introduction of secondary phase particles during FSP are still being studied, it is evident that
they have a significant impact. Understanding these mechanisms is vital for optimizing the
production process and achieving desired material properties. Moreover, FSP offers
significant advantages over traditional methods in terms of producing high-quality surface
composites. By further exploring these underlying mechanisms, researchers can harness the
full potential of FSP for creating high-performance surface composites.
Tool
Composite
Workpiece
Fig. Schematic diagram of surface composite formed via FSP
Impact of FSP-Specific Process Parameters
This section of the article focuses on the relationship between the process parameters and
their subsequent effect on the characteristic properties in friction stir processing (FSP)
operations, which is a relatively complex process because the near-surface property of the
modified material is dependent on a number of parameters. These parameters are fall into
the category of machine parameters, tool parameters and work material parameters as shown
in figure. The selection of appropriate process parameters is totally relied upon mechanical
properties of matrix materials. For instance, materials like steel, titanium, and copper have
higher hardness and yield strength with lower ductility due to that they require higher heat
input as compared to alloys like aluminum which having lower yield strength (YS), lower
hardness and higher ductility. Also, the peak temperature during processing is controlled by
the thermal properties of the material. In high thermal conductivity materials would allow
for more heat loss through conduction, more heat input would be needed to process them
without defects [32][33].
Fig - Classification of process parameters of FSP
Machining parameters
S. No.
Base, Tool
Tool
Operating
Material
design
parameters
parameters
Composite materials
Potential processing Routes for Aluminum matrix composite (AMC)
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